Effect of brazing temperature on microstructure and tensile strength ofγ-TiAl joint vacuum brazed with micro-nano Ti−Cu−Ni−Nb−Al−Hf filler

A novel micro-nano Ti−10Cu−10Ni−8Al−8Nb−4Zr−1.5Hf filler was used to vacuum braze Ti−47Al−2Nb−2Cr−0.15B alloy at 1160−1220°C for 30 min.The interfacial microstructure and formation mechanism of TiAl joints and the relationships among brazing temperature,interfacial microstructure and joint strength were emphatically investigated.Results show that the TiAl joints brazed at 1160 and 1180°C possess three interfacial layers and mainly consist ofα2-Ti3Al,τ3-Al3NiTi2 and Ti2Ni,but the brazing seams are no longer layered and Ti2Ni is completely replaced by the uniformly distributedτ3-Al3NiTi2 at 1200 and 1220°C due to the destruction ofα2-Ti3Al barrier layer.This transformation at 1200°C obviously improves the tensile strength of the joint and obtains a maximum of 343 MPa.Notably,the outward diffusion of Al atoms from the dissolution of TiAl substrate dominates the microstructure evolution and tensile strength of the TiAl joint at different brazing temperatures.

学前儿童森林意象感知模型与特征

森林是陆地生态系统的主体。研究学前儿童的森林意象感知,既有助于揭示其关于森林的已有经验,又可以为儿童森林教育的开展提供有益参考。本研究以来自湖南长沙、张家界与永州三所幼儿园的180名5~6岁儿童为研究对象,以景观意象素描法为研究方法,首先从语言知识、空间维度、自我定位、社会意义等方面建构了学前儿童森林意象感知模型,然后通过收集和分析被试儿童关于森林的绘画作品及对被试儿童的结构式访谈,探索其森林意象感知特征。结果显示在语言知识方面,“天空”“草本植物”“生物”“阔叶林”的出现频率较高,表明儿童偏好森林中的自然环境要素和当地原生植被类型;在空间维度方面,“侧视图”的出现频率最高,表明儿童偏好零散分布的林木、稀疏的林分或林缘景观;在自我定位方面,“周围的地方”出现的频率最高,表明儿童能够认识到人类与森林的互动,并将人类视为森林环境的一部分;在社会意义方面,“游憩空间”“生活世界”的出现频率较高,表明儿童较关注森林为人类提供游憩、居住场所的社会价值属性。总体来看,学前儿童对森林意象的感知呈现出主观、抽象与幻想的特征,同时也表现了对森林与人和谐相处的道德关怀。学前教育工作者应从儿童森林意象感知特征出发,加强对儿童的森林教育,引导儿童真正走进森林,培养儿童热爱、尊重、保护自然的情怀。

Empirical research on construction of a measurement framework for tourism carbon emission in China

According to the logic process of carbon reduction in China which arises from the measurement to reduction,from reduction to offsetting,the measurement of carbon emission in the tourism industry was the first and key step.Based on the life cycle assessment theory and input-output analysis,this article used economic and environmental measurement technologies,The System of National Accounting(SNA),Tourism Satellite Account(TSA),System of Integrated Environment and Economic Accounting(SEEA),and so on,and built up a top-down carbon emission analysis framework for the tourism industry and estimated carbon emission of the tourism industry in China in 2007.The finding showed that the total carbon emission of the tourism industry in China in 2007 was 169.78 million tons,covering 2.71%of carbon emission of all industries in China in 2007,and 2.44%of the total carbon emission in China in 2007.The direct carbon emission of the tourism industry in China in 2007 was 73.56 million tons,including transportation(50.14 million tons),sightseeing(1.33 million tons),lodging(4.19 million tons),accommodation(4.73 million tons),shopping(8.14 million tons),entertainment(0.67 million tons),communication(0.45 million tons),and others(3.90 million tons).The indirect carbon emission of the tourism industry in China in 2007 was 96.23 million tons,mostly contributed by coking,gas,and petroleum processing industries,transportation and warehousing industry,machinery and equipment manufacturing industry,and food manufacturing and tobacco processing industry,which covered 57%.

Ab initio Predictions of Structural and Thermodynamic Properties of Zr2AIC Under High Pressure and High Temperature

The structural and thermodynamic properties of Zr2A1C at high pressure and high temper- ature are investigated by first principles density functional theory method. The calculated lattice parameters of Zr2A1C are in good agreement with the available theoretical data. The pressure dependences of the elastic constants, bulk modulus, shear modulus, Young＇s mod- ulus, and Vickers hardness of Zr2A1C are successfully obtained. The elastic anisotropy is examined through the computation of the direction dependence of Young＇s modulus. By using the quasiharmonic Debye model, the thermodynamic properties including the Debye temperature, heat capacity, volume thermal expansion coefficient, and Griineisen parameter at high pressure and temperature are predicted for the first time.

二氢乳清酸脱氢酶的结构特征和催化循环研究进展

二氢乳清酸脱氢酶是黄素依赖的线粒体酶,它催化嘧啶从头合成的第4步反应,将二氢乳清酸氧化为乳清酸。通过选择性抑制二氢乳清酸脱氢酶,从而抑制嘧啶的合成,已被开发用于治疗癌症、自身免疫性疾病、细菌或病毒感染以及寄生虫疾病等。抑制剂的开发需详细了解二氢乳清酸脱氢酶的结构特征和催化循环机制。因此,文中主要从这两个方面进行了综述,并展望了该酶的抑制剂在临床应用中的前景。